1. Field of Invention
The invention relates to an electro-optical device for performing display in information instruments such as televisions or computers. More specifically, the invention relates to a driving device for driving electro-optical elements such as organic electroluminescent elements.
2. Description of Related Art
In recent, since organic EL display devices have features as light weight, small thickness, high brightness, and wide viewing angle, the organic EL display devices had been paid attention to as monitor displays of portable information instruments, such as mobile phones. A typical active matrix organic EL display device has a construction that images are displayed through a plurality of display pixels arranged in a matrix. In the display pixels, a pixel circuit can be provided every pixel that is a minimum unit for display. The pixel circuits are circuits for controlling currents or voltages to be supplied to electro-optical elements.
In such an organic EL display device, a plurality of scanning lines are arranged along the rows of the display pixels, a plurality of data lines are arranged along the columns of the display pixels, and a plurality of pixel switches are arranged in the vicinity of intersections between the scanning lines and the data lines. Each display pixel comprises at least an organic EL element, a driving transistor connected in series to the organic EL element between a pair of power terminals, and a storage capacitor for holding a gate voltage of the driving transistor. The selecting switch of each pixel is electrically conducted in response to a scan signal supplied from the corresponding scanning line, and applies an image signal (voltage or current) supplied from the corresponding data line directly or a gray scale voltage as a result of correcting deviation in characteristics of the pixel circuit to the gate electrode of the driving transistor. The driving transistor supplies a driving current corresponding to the gray scale voltage to the organic EL element.
The organic EL element has a structure in which a light-emitting layer that is a thin film including a fluorescent organic compound of red, green or blue is inserted and held between a common electrode (cathode) and a pixel electrode (anode), generates exitons by injecting electrons and holes in the light-emitting layer and recombining them, and emits light by light emission occurring due to deactivation of the exitons. In a bottom emission type organic EL element, the electrode thereof is a transparent electrode made of ITO, etc., and the common electrode (cathode) is formed out of a reflecting electrode in which a resistance of an alkali metal is decreased using metal such as aluminum, etc. As a result, with an applied voltage of 10 V or less, a luminance of about 100 to 100,000 cd/m2 can be obtained from the organic EL element itself.
Each pixel circuit of the aforementioned organic EL display device can include thin film transistors (TFTs) as active elements, as disclosed in Japanese Unexamined Patent Application Publication No. 5-107561. The thin film transistor is formed out of, for example, a low-temperature polysilicon TFT.